1. Field of the Invention
The present invention relates to an occupant protection system for a vehicle, and, more particularly, to a collision adaptive occupant protection system for a vehicle that strains a seat belt so as thereby to restrain the occupant's body tightly against the back of a seat.
2. Description of Related Art
In order to protect occupants of a vehicle from injuries upon an occurrence of a collision of the vehicle, a seat is equipped with a seat belt that restrains an occupant body against the back of the seat. There are different types of collisions, namely a front-end collision that a vehicle runs into an obstacle such as a vehicle ahead and a rear-end collision that a vehicle is run into from behind. The rear-end collision is hardly predictable for occupants, in particular a driver, of the vehicle who sit facing the front. Therefore, there have been proposed various collision adaptive seat belt systems that can strongly strain the seat belt instantaneously when a rear-end collision is predicted so as thereby to restrain the occupant's body tightly against the back of the seat. Such a collision adaptive seat belt system is constructed so as to predict an occurrence of a rear-end collision of the vehicle with a following vehicle approaching the vehicle from the rear on the basis of a space distance between the vehicle and the following vehicle or a relative speed between the two vehicles that is detected by a monitor such as radar.
One of the collision adaptive seat beat systems described in, for example, Japanese Unexamined Patent Application No. 9-175327 performs rear-end collision prediction on the basis of a space distance between a vehicle and a following vehicle approaching the vehicle from the rear that is detected by a monitor and predetermined upper and lower critical allowable times before a crash. When an allowable time before a crash that is calculated on the basis of the space distance is between the upper and lower critical allowable times, it is judged that a rear-end collision is within the range of possibility, then, the seat belt is comparatively strongly strained as a warning of a crash provided for the occupant. Further, when the allowable time exceeds the lower critical allowable time, it is predicted that a rear-end collision will occurs, then, the seat belt is strongly strained so as thereby to tightly restrain the occupant's body against the back of the seat for preparation for a crash.
The prior art collision adaptive seat belt system possibly raises a false alarm of a predicted collision although no one is tailgating the vehicle. The false alarm is conceivable to be raised resulting from that the monitor confuses rear stationary traffic devices such as protective wall panel and guard rail with a vehicle approaching from the rear. More specifically, in order for the seat belt system to raise an alarm of a collision and to strain the seat belt to put the occupant prepared for a crash, it is conventional to provide the vehicle with a monitor such as distance sensor that emits a detecting wave behind the vehicle and receives an echo from a following vehicle so as to detect a space distance to the following vehicle on the basis of the echo. When making a turn in a sharp bent lane or making a left or right turn at an intersection, it is so often that the vehicle changes its position so as to gradually decrease a distance to a stationary protective wall panel or a guard rail as a rear stationary traffic device in an aiming direction of the distance sensor. As a result, the collision adaptive seat beat system misidentifies the protective wall panel or the guard rail as a vehicle gradually approaching from the rear.
In order to avoid a misidentification of the rear stationary traffic device as a following vehicle approaching the vehicle from the rear, it is conceivably effective to enable the collision adaptive seat beat system to distinguish a rear stationary traffic device from a vehicle. However, it is quite hard to distinguish the rear stationary traffic device from a vehicle. Discrimination between these two objects thrusts imaging a rear field behind the vehicle and processing the image onto the collision adaptive seat beat system. This levies a high cost on the collision predictable seat beat system.